Etymology
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spectrum (n.)
1610s, "apparition, specter," from Latin spectrum (plural spectra) "an appearance, image, apparition, specter," from specere "to look at, view" (from PIE root *spek- "to observe"). Meaning "visible band showing the successive colors, formed from a beam of light passed through a prism" first recorded 1670s. Figurative sense of "entire range (of something)" is from 1936.
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spectro- 
word-forming element meaning "of or by a spectroscope," also "of radiant energy," from combining form of spectrum.
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specter (n.)
c. 1600, "frightening ghost," from French spectre "an image, figure, ghost" (16c.), from Latin spectrum "appearance, vision, apparition" (see spectrum). Figurative sense "object of dread" is from 1774.
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spectral (adj.)
1718, "capable of seeing spectres;" 1815, "ghostly;" from spectre + -al (1). Meaning "pertaining to a spectrum" is 1832, from stem of spectrum + -al (1). Related: Spectrally.
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*spek- 
Proto-Indo-European root meaning "to observe."

It forms all or part of: aspect; auspex; auspices; auspicious; bishop; circumspect; conspicuous; despicable; despise; episcopal; especial; espionage; espy; expect; frontispiece; gyroscope; haruspex; horoscope; inspect; inspection; inspector; introspect; introspection; perspective; perspicacious; perspicacity; prospect; prospective; respect; respite; retrospect; scope; -scope; scopophilia; -scopy; skeptic; species; specimen; specious; spectacle; spectacular; spectrum; speculate; speculation; speculum; spice; spy; suspect; suspicion; suspicious; telescope.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit spasati "sees;" Avestan spasyeiti "spies;" Greek skopein "behold, look, consider," skeptesthai "to look at," skopos "watcher, one who watches;" Latin specere "to look at;" Old High German spehhon "to spy," German spähen "to spy."
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spectrogram (n.)
"photograph of a spectrum," 1890, from spectro- + -gram.
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black-light (n.)
"light rays beyond the visible spectrum," 1927, from black (adj.) + light (n.).
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infra-red (adj.)
also infrared, 1873, "below the red" (in the spectrum), from infra- + red (adj.1). As a noun, also from 1873.
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ultraviolet (adj.)
"beyond the violet end of the visible spectrum," 1840, from ultra- "beyond" + violet. Ultra-red (1870) was a former name for what now is called infra-red.
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thallium (n.)
rare metallic element, 1861, Modern Latin, from Greek thallos "young shoot, green branch" (see thallus) + element name ending -ium. So called by its discoverer, Sir William Crookes (1832-1919), from the green line in its spectrum by which he detected it. Related: Thallic.
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